U.S. patent number 10,323,301 [Application Number 14/866,070] was granted by the patent office on 2019-06-18 for hydrogen storing alloy, electrode, and nickel-hydrogen storage battery.
This patent grant is currently assigned to GS Yuasa International Ltd.. The grantee listed for this patent is GS Yuasa International Ltd.. Invention is credited to Manabu Kanemoto, Mitsuhiro Kodama, Daisuke Okuda.
United States Patent |
10,323,301 |
Okuda , et al. |
June 18, 2019 |
Hydrogen storing alloy, electrode, and nickel-hydrogen storage
battery
Abstract
Provided is a hydrogen storing alloy represented by the general
formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.3<a<0.6; 0<b<0.16; 0.1<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.2<x<3.5; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co). Also provided is a hydrogen storing alloy represented by the
general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.1<a<0.25; 0.1<b<0.2; 0.02<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.6<x<3.7; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co). Further provided is a nickel-metal hydride rechargeable
battery including a negative electrode containing the hydrogen
storing alloy.
Inventors: |
Okuda; Daisuke (Kyoto,
JP), Kanemoto; Manabu (Kyoto, JP), Kodama;
Mitsuhiro (Kyoto, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
GS Yuasa International Ltd. |
Kyoto-shi, Kyoto |
N/A |
JP |
|
|
Assignee: |
GS Yuasa International Ltd.
(Kyoto, JP)
|
Family
ID: |
55583793 |
Appl.
No.: |
14/866,070 |
Filed: |
September 25, 2015 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20160090643 A1 |
Mar 31, 2016 |
|
Foreign Application Priority Data
|
|
|
|
|
Sep 30, 2014 [JP] |
|
|
2014-200689 |
Sep 30, 2014 [JP] |
|
|
2014-200690 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C
19/03 (20130101); H01M 10/345 (20130101); H01M
4/383 (20130101); C22C 19/007 (20130101); C22C
1/0433 (20130101); B22F 9/002 (20130101); H01M
10/30 (20130101); Y02E 60/10 (20130101); B22F
2998/00 (20130101); B22F 2998/10 (20130101); B22F
2998/10 (20130101); B22F 3/14 (20130101); B22F
9/04 (20130101); B22F 1/0059 (20130101); B22F
1/0074 (20130101); B22F 7/04 (20130101); B22F
2998/10 (20130101); B22F 3/14 (20130101); B22F
9/04 (20130101); B22F 1/0003 (20130101); B22F
1/0074 (20130101); B22F 7/04 (20130101); B22F
2998/10 (20130101); B22F 3/14 (20130101); B22F
9/04 (20130101); C22C 1/05 (20130101); B22F
1/0074 (20130101); B22F 7/04 (20130101); B22F
2998/00 (20130101); B22F 9/082 (20130101); B22F
9/026 (20130101); B22F 9/008 (20130101) |
Current International
Class: |
C22C
19/03 (20060101); C22C 1/04 (20060101); B22F
9/00 (20060101); H01M 10/34 (20060101); C22C
19/00 (20060101); H01M 10/30 (20060101); H01M
4/38 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
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11323469 |
|
Nov 1999 |
|
JP |
|
2007291474 |
|
Nov 2007 |
|
JP |
|
2008071687 |
|
Mar 2008 |
|
JP |
|
2009176712 |
|
Aug 2009 |
|
JP |
|
2011021262 |
|
Feb 2011 |
|
JP |
|
2011082129 |
|
Apr 2011 |
|
JP |
|
2012067357 |
|
Apr 2012 |
|
JP |
|
201299250 |
|
May 2012 |
|
JP |
|
2014114476 |
|
Jun 2014 |
|
JP |
|
2012057351 |
|
May 2012 |
|
WO |
|
Other References
Machine translation of WO2012057351A1 (Japanese language document
published May 3, 2012). cited by examiner.
|
Primary Examiner: Kessler; Christopher S
Attorney, Agent or Firm: Rankin, Hill & Clark LLP
Claims
What is claimed is:
1. A hydrogen storing alloy represented by the general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.3<a<0.6; 0<b<0.16; 0.1<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.2<x<3.5; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co), wherein the content of La in RE is 0.3 or more in terms of a
molar amount based on the total amount of RE, Sm and Mg.
2. The hydrogen storing alloy according to claim 1, wherein RE
contains Nd and/or Pr.
3. The hydrogen storing alloy according to claim 2, wherein the
hydrogen storing alloy contains 70% by mass or more of a
Ce.sub.2Ni.sub.7 phase as a crystal phase of the alloy.
4. The hydrogen storing alloy according to claim 1, wherein the
hydrogen storing alloy contains 70% by mass or more of a
Ce.sub.2Ni.sub.7 phase as a crystal phase of the alloy.
5. An electrode comprising the hydrogen storing alloy according to
claim 1.
6. A nickel-metal hydride rechargeable battery comprising the
electrode according to claim 5 as a negative electrode.
7. A hydrogen storing alloy represented by the general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.1.ltoreq.a.ltoreq.0.25; 0.1<b<0.2;
0.02<cx<0.2; 0.ltoreq.dx.ltoreq.0.1; 3.6.ltoreq.x.ltoreq.3.7;
RE is at least two elements selected from the group consisting of a
rare earth element other than Sm, and Y, and essentially contains
La and contains Nd and/or Pr; and M is Mn and/or Co).
8. The hydrogen storing alloy according to claim 7, wherein the
content of La in RE is 0.6 or more in terms of a molar amount based
on the total amount of RE, Sm and Mg.
9. The hydrogen storing alloy according to claim 8, wherein the
hydrogen storing alloy contains 80% by mass or more of a
Pr.sub.5Co.sub.19 phase and Ce.sub.5Co.sub.19 phase as a crystal
phase of the alloy.
10. The hydrogen storing alloy according to claim 7, wherein the
hydrogen storing alloy contains 80% by mass or more of a
Pr.sub.5Co.sub.19 phase and Ce.sub.5Co.sub.19 phase as a crystal
phase of the alloy.
11. An electrode comprising the hydrogen storing alloy according to
claim 7.
12. A nickel-metal hydride rechargeable battery comprising the
electrode according to claim 11 as a negative electrode.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese patent applications
No. 2014-200689, filed on Sep. 30, 2014, and No. 2014-200690, filed
on Sep. 30, 2014, which are incorporated by reference.
FIELD
The present invention relates to a novel hydrogen storing alloy, an
electrode containing the hydrogen storing alloy, and a nickel-metal
hydride rechargeable battery including the electrode.
BACKGROUND
Nickel-metal hydride rechargeable batteries are widely used as
power sources for small electronic devices such as digital cameras
and notebook personal computers due to their high energy density,
and as power sources for electric automobiles and hybrid
automobiles due to their suitability for high-power applications
and excellent safety.
Such a nickel-metal hydride rechargeable battery normally includes
a nickel electrode containing a positive active material mainly
composed of nickel hydroxide, a negative electrode containing a
hydrogen storing alloy as a main material, a separator, and an
alkali electrolyte solution. Among these battery constituent
materials, particularly the hydrogen storing alloy as a main
material of the negative electrode significantly affects
performance of the nickel-metal hydride rechargeable battery, such
as a discharge capacity and an energy density. As the hydrogen
storing alloy, various kinds of alloys have been heretofore
examined.
Particularly, for the purpose of increasing the capacity of a
nickel-metal hydride rechargeable battery, an attempt has been made
to use a rare earth-Mg--Ni-based hydrogen storing alloy for a
negative electrode (see Japanese Patent Laid-open Publication No.
11-323469).
However, the rare earth-Mg--Ni-based alloy has the problem that it
is extremely poor in durability when used as a negative electrode
of a battery. This is ascribable to occurrence of distortion among
a plurality of crystal phases included in the alloy due to
absorption and release of hydrogen associated with
charge-discharge. There is also the problem that repetition of
charge-discharge accelerates pulverization of the hydrogen storing
alloy, leading to deterioration of durability. Further, the
hydrogen storing alloy has the problem that it is easily corroded
when the battery is stored under a high-temperature atmosphere, or
charge-discharge is repeated.
In order to solve these problems, studies have been repeatedly
conducted heretofore for optimizing the ratio of substitution
elements or crystal phases (see, for example, Japanese Patent
Laid-open Publication No. 2007-291474, Japanese Patent Laid-open
Publication No. 2008-71684, Japanese Patent Laid-open Publication
No. 2009-176712, Japanese Patent Laid-open Publication No.
2011-21262, Japanese Patent Laid-open Publication No. 2011-82129,
Japanese Patent Laid-open Publication No. 2012-67357 and Japanese
Patent Laid-open Publication No. 2014-114476), but an alloy having
satisfactory corrosion resistance and durability has not been
obtained yet.
SUMMARY
The following presents a simplified summary of the invention
disclosed herein in order to provide a basic understanding of some
aspects of the invention. This summary is not an extensive overview
of the invention. It is intended to neither identify key or
critical elements of the invention nor delineate the scope of the
invention. Its sole purpose is to present some concepts of the
invention in a simplified form as a prelude to the more detailed
description that is presented later.
An object of the present invention is to provide a hydrogen storing
alloy excellent in corrosion resistance and durability, and a
nickel-metal hydride rechargeable battery which includes the
hydrogen storing alloy and is excellent in cycle life.
The aspect of the present invention employs the following means for
achieving the object. (1) A hydrogen storing alloy represented by
the general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.3<a<0.6; 0<b<0.16; 0.1<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.2<x<3.5; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co).
DESCRIPTION OF EMBODIMENTS
The aspect of the present invention employs the following means for
achieving the object. (1) A hydrogen storing alloy represented by
the general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.3<a<0.6; 0<b<0.16; 0.1<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.2<x<3.5; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co). (2) The hydrogen storing alloy according to (1), wherein the
content of La in RE is 0.3 or more in terms of a molar amount based
on the total amount of RE, Sm and Mg. (3) A hydrogen storing alloy
represented by the general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.1<a<0.25; 0.1<b<0.2; 0.02<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.6<x<3.7; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co). (4) The hydrogen storing alloy according to (3), wherein the
content of La in RE is 0.6 or more in terms of a molar amount based
on the total amount of RE, Sm and Mg. (5) The hydrogen storing
alloy according to (1), wherein RE contains Nd and/or Pr. (6) The
hydrogen storing alloy according to (2), wherein RE contains Nd
and/or Pr. (7) The hydrogen storing alloy according to (3), wherein
RE contains Nd and/or Pr. (8) The hydrogen storing alloy according
to (4), wherein RE contains Nd and/or Pr. (9) The hydrogen storing
alloy according to (1), wherein the hydrogen storing alloy contains
70% by mass or more of a Ce.sub.2Ni.sub.7 phase as a crystal phase
of the alloy. (10) The hydrogen storing alloy according to (2),
wherein the hydrogen storing alloy contains 70% by mass or more of
a Ce.sub.2Ni.sub.7 phase as a crystal phase of the alloy. (11) The
hydrogen storing alloy according to (5), wherein the hydrogen
storing alloy contains 70% by mass or more of a Ce.sub.2Ni.sub.7
phase as a crystal phase of the alloy. (12) The hydrogen storing
alloy according to (6), wherein the hydrogen storing alloy contains
70% by mass or more of a Ce.sub.2Ni.sub.7 phase as a crystal phase
of the alloy. (13) The hydrogen storing alloy according to (3),
wherein the hydrogen storing alloy contains 80% by mass or more of
a Pr.sub.5Co.sub.19 phase and Ce.sub.5Co.sub.19 phase as a crystal
phase of the alloy. (14) The hydrogen storing alloy according to
(4), wherein the hydrogen storing alloy contains 80% by mass or
more of a Pr.sub.5Co.sub.19 phase and Ce.sub.5Co.sub.19 phase as a
crystal phase of the alloy. (15) The hydrogen storing alloy
according to (7), wherein the hydrogen storing alloy contains 80%
by mass or more of a Pr.sub.5Co.sub.19 phase and Ce.sub.5Co.sub.19
phase as a crystal phase of the alloy. (16) The hydrogen storing
alloy according to (8), wherein the hydrogen storing alloy contains
80% by mass or more of a Pr.sub.5Co.sub.19 phase and
Ce.sub.5Co.sub.19 phase as a crystal phase of the alloy. (17) An
electrode including the hydrogen storing alloy according to (1).
(18) An electrode including the hydrogen storing alloy according to
(3). (19) A nickel-metal hydride rechargeable battery including the
electrode according to (17) as a negative electrode. (20) A
nickel-metal hydride rechargeable battery including the electrode
according to (18) as a negative electrode. (First Embodiment)
The first embodiment of the present invention will be described in
detail below.
The hydrogen storing alloy according to the first embodiment of the
present invention is a hydrogen storing alloy represented by the
general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.- x
(where 0.3<a<0.6; 0<b<0.16; 0.1<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.2<x<3.5; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co).
In the hydrogen storing alloy according to the first embodiment of
the present invention, B/A (numerical value range of x) is more
than 3.2 and less than 3.5 as shown in the above-mentioned general
formula. When a hydrogen storing alloy in which B/A falls within
the above-mentioned range, the content of a Ce.sub.2Ni.sub.7 phase
is 70% by mass or more, so that corrosion resistance is improved.
In either the case where B/A is 3.2 or less or the case where B/A
is 3.5 or more, corrosion resistance is deteriorated.
It has become apparent that when an alloy having a specific B/A
ratio as described above is substituted with a predetermined amount
of Sm (samarium), a hydrogen storing alloy having high corrosion
resistance (durability) can be obtained. For improving corrosion
resistance, the Sm substitution amount is more than 0.3 and less
than 0.6 in terms of a molar amount based on the total amount of
Sm, RE (at least one element selected from the group consisting of
a rare earth element other than Sm, and Y) and Mg. The Sm
substitution amount is preferably 0.5 or less. In either the case
where the Sm substitution amount is 0.3 or less or the case where
the Sm substitution amount is 0.6 or more, corrosion resistance is
deteriorated.
For the mechanism in which corrosion resistance is improved when
the Sm substitution amount is more than 0.3 and less than 0.6, it
is thought that when the alloy is substituted with a predetermined
amount of Sm, the Ce.sub.2Ni.sub.7 phase is stably generated, so
that the crystal structure is uniquely stabilized, and thus
distortion of the overall alloy can be prevented.
In the hydrogen storing alloy according to the first embodiment of
the present invention, RE is at least one element selected from the
group consisting of a rare earth element other than Sm, and Y, and
essentially contains La as a main component. The content of La is
preferably 0.3 or more, more preferably 0.3 to 0.5 in terms of a
molar amount based on the total amount of Sm, RE (at least one
element selected from the group consisting of a rare earth element
other than Sm, and Y) and Mg. In the case where La is not
contained, corrosion resistance is not improved even when the
above-mentioned general formula is satisfied. Examples of RE
excluding La include Nd, Pr, Ce, Dy and Gd, and among them, Nd
and/or Pr are preferred because corrosion resistance is improved
when these elements are contained.
In the hydrogen storing alloy according to the first embodiment of
the present invention, the content of Mg is less than 0.16 in terms
of a molar amount based on the total amount of Sm, RE (at least one
element selected from the group consisting of a rare earth element
other than Sm, and Y) and Mg for improving corrosion resistance.
The content of Mg is preferably less than 0.15. When the content of
Mg is 0.16 or more, corrosion resistance is deteriorated.
In the hydrogen storing alloy according to the first embodiment of
the present invention, the main component that forms the B site is
Ni, and when along with Ni, Al is contained in an amount of more
than 0.1 and less than 0.2 in terms of molar amount multiplied by
X, a hydrogen storing alloy having improved corrosion resistance is
obtained. In either the case where the Al amount is 0.1 or less or
the case where the Al amount is 0.2 or more, corrosion resistance
is deteriorated. Along with Al, Mn and/or Co can be contained in an
amount of 0.1 or less in terms of a molar amount multiplied by X
for improving corrosion resistance.
In the hydrogen storing alloy according to the first embodiment of
the present invention, corrosion resistance is improved when the
crystal phase contains 70% by mass or more of a Ce.sub.2Ni.sub.7
phase. In addition to the Ce.sub.2Ni.sub.7 phase, a
Gd.sub.2Co.sub.7 phase, a Pr.sub.5Co.sub.19 phase, a
Ce.sub.5Co.sub.19 phase, a CaCu.sub.5 phase or the like may be
contained.
A method for producing the hydrogen storing alloy of this
embodiment includes, for example, a melting step of melting an
alloy raw material blended so as to achieve a predetermined
composition ratio as described above; a cooling step of solidifying
the molten alloy raw material; an annealing step of annealing the
cooled alloy; and a grinding step of grinding the alloy.
The steps will be described more in detail. First, a predetermined
amount of a raw material ingot (alloy raw material) is weighed
based on the chemical composition of an intended hydrogen storing
alloy.
In the melting step, the alloy raw material is added in crucible,
and melted at 1000.degree. C. or higher in an inert gas atmosphere
or in vacuum using a high-frequency melting furnace etc. The upper
limit of the melting temperature is about 2000.degree. C. For
example, the alloy raw material is heated at 1200 to 1600.degree.
C. to be melted.
In the cooling step, the molten alloy raw material is cooled to be
solidified. The cooling speed may be either a slow cooling speed or
1000 K/second or more (also referred to as a rapid cooling speed),
but it is preferred to use a rapid cooling speed. Rapid cooling at
1000 K/second has an effect of causing the alloy composition to
become fine and heterogeneous. The cooling speed can be set to
1000000 K/second or less.
Specifically, a water-cooling mold method, a melt spinning method
with a cooling speed of 100000 K/second or more, a gas atomizing
method with a cooling speed of about 10000 K/second, or the like
can be used as the cooling method.
In the annealing step, the alloy is heated at 860.degree. C. or
higher and 1000.degree. C. or lower in a compressed state under an
inert gas atmosphere using, for example, an electric furnace etc.
Preferably, the alloy is heated at 930 to 975.degree. C. The
compression condition is preferably 0.2 MPa (gauge pressure) or
more and 1.0 MPa (gauge pressure) or less. The treatment time is
the annealing step is preferably 3 hours or more and 50 hours or
less.
The grinding step may be carried out either before or after
annealing, but since the surface area is increased by grinding, it
is desirable to carry out the grinding step after the annealing
step for preventing oxidation of the surface of the alloy.
Preferably, grinding is performed in an inert atmosphere for
preventing oxidation of the surface of the alloy.
As grinding means, for example, mechanical grinding, hydrogenation
grinding or the like is used, and it is preferred to perform
grinding in such a manner that the particle size of hydrogen
storing alloy particles after grinding is approximately 20 to 70
.mu.m.
The application of the hydrogen storing alloy according to the
first embodiment of the present invention is not particularly
limited, and it can be used in various applications such as
nickel-metal hydride rechargeable batteries, fuel batteries, and
fuel tanks for hydrogen automobiles. Particularly, the hydrogen
storing alloy is suitably used for a negative active material of a
nickel-metal hydride rechargeable battery. A nickel-metal hydride
rechargeable battery including a negative electrode containing the
hydrogen storing alloy according to the first embodiment of the
present invention as described above is also one aspect of the
present invention.
The nickel-metal hydride rechargeable battery according to the
first embodiment of the present invention further includes a
positive electrode (nickel electrode) containing a positive active
material mainly composed of nickel hydroxide, a separator, an
alkali electrolyte solution and so on in addition to the negative
electrode containing as a negative active material the hydrogen
storing alloy according to the first embodiment of the present
invention.
The negative electrode contains as a negative active material the
hydrogen storing alloy according to the first embodiment of the
present invention. For example, the hydrogen storing alloy
according to the first embodiment of the present invention is
blended in the negative electrode in the form of a powdered
hydrogen storing alloy.
The negative electrode may contain a conducting agent, a binder
(including a thickener) and so on in addition to the hydrogen
storing alloy powder.
Examples of the conducting agent include carbon-based conducting
agents such as natural graphite (scaly graphite, scalelike, earthy
graphite and the like), artificial graphite, carbon black,
acetylene black, ketjen black, carbon whiskers, carbon fibers and
vapor phase growth carbon; and metal-based conducting agents
composed of powders, fibers and the like of metals such as nickel,
cobalt and copper. These conducting agents may be used alone, or
may be used in combination of two or more thereof. A rare earth
oxide such as yttrium oxide may be contained as an anticorrosive
agent.
The blending amount of the conducting agent is preferably 0.1 to 10
parts by mass, more preferably 0.2 to 5 parts by mass based on 100
parts by mass of the hydrogen storing alloy powder. When the
blending amount of the conducting agent is less than 0.1 part by
mass, it is difficult to achieve sufficient conductivity. On the
other hand, when the blending amount of the conducting agent is
more than 10 parts by mass, the discharge capacity improving effect
may be insufficient.
Examples of the binder include polyolefin-based resins such as
polytetrafluoroethylene (PTHE), polyethylene and polypropylene,
ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene
butadiene rubber, fluororubber, polyvinyl alcohol, methyl
cellulose, carboxymethyl cellulose and xanthan gum. These binders
may be used alone, or may be used in combination of two or more
thereof.
The blending amount of the binder is preferably 0.1 to 1.0 parts by
mass, more preferably 0.5 to 1.0 parts by mass based on 100 parts
by mass of the hydrogen storing alloy powder. When the blending
amount of the binder is less than 0.1 part by mass, it is difficult
to achieve sufficient thickenability. On the other hand, when the
blending amount of the binder is more than 1.0 part by mass, the
performance of the electrode may be deteriorated.
The positive electrode is, for example, an electrode containing as
a positive active material a nickel hydroxide composite oxide
formed by mixing zinc hydroxide or cobalt hydroxide with nickel
hydroxide as a main component. As the nickel hydroxide composite
oxide, one that is uniformly dispersed by a coprecipitation method
is suitably used.
Preferably, the positive electrode contains an additive for
improving electrode performance in addition to the nickel hydroxide
composite oxide. The additive is, for example, a conductivity
modifier such as cobalt hydroxide or cobalt oxide. Alternatively,
the nickel hydroxide composite oxide may be coated with cobalt
hydroxide, or the nickel hydroxide composite oxide may be partially
oxidized by oxygen, an oxygen-containing gas,
K.sub.2S.sub.2O.sub.8, hypochlorous acid or the like.
As the additive, a compound containing a rare earth element such as
Y or Yb, or an oxygen overvoltage improving substance such as a
compound containing Ca can also be used. The rare earth element
such as Y or Yb is partially dissolved and disposed on the surface
of the negative electrode, and is therefore expected to exhibit an
effect of suppressing corrosion of a negative active material.
The positive electrode may further contain the conducting agent,
binder and so on as in the case of the negative electrode.
The above-mentioned positive electrode and negative electrode can
be produced by, for example, adding the conducting agent, binder
and so on to each active material as necessary, then mixing the
mixture together with water or an organic solvent such as an
alcohol or toluene to obtain a paste, applying the paste to a
conductive support, drying the paste, and performing roll
molding.
Examples of the conductive support include steel plates, and plated
steel plates obtained by subjecting a steel plate to plating with a
metal material such nickel. Examples of the form of the conductive
support include foams, molded products of fiber groups,
three-dimensional substrates subjected to irregularity processing,
and two-dimensional substrates such as punching plates. Among these
conductive supports, a foam made from nickel excellent in corrosion
resistance to alkalis and resistance to oxidation and composed of a
porous structure that is a structure excellent in current
collection property is preferred as a conductive support for a
positive electrode. On the other hand, as a conductive support for
a negative electrode, a pierced steel plate obtained by subjecting
to nickel plating an iron foil which is inexpensive and excellent
in conductivity.
The thickness of the conductive support is preferably 30 to 100
.mu.m, more preferably 40 to 70 .mu.m. When the thickness of the
conductive support is less than 30 .mu.m, productivity may be
deteriorated. On the other hand, when the thickness of the
conductive support is more than 100 .mu.m, the discharge capacity
may be insufficient.
When the conductive support is porous, the inner diameter thereof
is preferably 0.8 to 2 .mu.m, more preferably 1 to 1.5 .mu.m. When
the inner diameter is less than 0.8 .mu.m, productivity may be
deteriorated. On the other hand, when the inner diameter is more
than 2 .mu.m, the retention capacity of the hydrogen storing alloy
may be insufficient.
Examples of the method for applying a paste for each electrode to
the conductive support include roller coating using an applicator
roll etc., screw coating, blade coating, spin coating and bar
coating.
Examples of the separator include a porous films and nonwoven
fabrics made from a polyolefin-based resin such as polyethylene or
polypropylene, an acryl, a polyamide or the like.
The weight per unit area of the separator is preferably 40 to 100
g/m.sup.2. When the weight per unit area is less than 40 g/m.sup.2,
a short circuit or deterioration of self discharge performance may
occur. On the other hand, when the weight per unit area is more
than 100 g/m.sup.2, the battery capacity tends to decrease because
the ratio of the separator per unit volume increases. The air
permeability of the separator is preferably 1 to 50 cm/sec. When
the air permeability is less than 1 cm/sec, the battery internal
pressure may be excessively high. On the other hand, when the air
permeability is more than 50 cm/sec, a short circuit or
deterioration of self discharge performance may occur. Further, the
average fiber diameter of the separator is preferably 1 to 20
.mu.m. When average fiber diameter is less than 1 .mu.m, the
strength of the separator may be reduced, leading to an increase in
defect rate in a battery assembling step. On the other hand, when
the average fiber diameter is more than 20 .mu.m, a short circuit
or deterioration of self discharge performance may occur.
Preferably, the separator is subjected to a hydrophilization
treatment at the fiber surface thereof. Examples of the
hydrophilization treatment include a sulfonation treatment, a
corona treatment, a fluorine gas treatment and a plasma treatment.
Particularly, a separator subjected to a sulfonation treatment at
the fiber surface is preferred because it has a high capability of
adsorbing impurities such as NO.sub.3.sup.-, NO.sub.2.sup.- and
NH.sub.3.sup.- and eluted elements from a negative electrode, which
cause a shuttle phenomenon, and therefore exhibits a high self
discharge suppressing effect.
Examples of the alkali electrolyte solution include alkaline
aqueous solutions containing potassium hydroxide, sodium hydroxide,
lithium hydroxide or the like. The alkali electrolyte solutions may
be used alone, or may be used in combination of two or more
thereof.
The concentration of the alkali electrolyte solution is preferably
9.0 M or less, more preferably 5.0 to 8.0 M in terms of the sum of
ion concentrations.
To the alkali electrolyte solution may be added various additives
for improvement of the oxygen overvoltage at the positive
electrode, improvement of corrosion resistance of the negative
electrode and improvement of self discharge. Examples of such
additives include oxides and hydroxides such as those of Y, Yb, Er,
Ca and Zn. These additives may be used alone, or may be used in
combination of two or more thereof.
In the case where the nickel-metal hydride rechargeable battery
according to the first embodiment of the present invention is an
open-type nickel-metal hydride rechargeable battery, the battery
can be produced by, for example, sandwiching a negative electrode
in a positive electrode with a separator interposed therebetween,
injecting an alkali electrolyte solution with the electrodes fixed
in such a manner as to apply a predetermined pressure to the
electrodes, and assembling an open-type cell.
On the other hand, in the case where the nickel-metal hydride
rechargeable battery according to the first embodiment of the
present invention is a closed-type nickel-metal hydride
rechargeable battery, the battery can be produced by injecting an
alkali electrolyte solution before or after laminating a positive
electrode, a separator and a negative electrode, and sealing the
electrolyte solution with an exterior material. In a closed-type
nickel-metal hydride rechargeable battery formed by winding a power
generating element in which a positive electrode and a negative
electrode are laminated with a separator interposed therebetween,
it is preferred to inject an alkali electrolyte solution in the
power generating element before or after winding the power
generating element. The method for injecting an alkali electrolyte
solution is not particularly limited, and the alkali electrolyte
solution may be injected at normal pressure, or for example, a
vacuum impregnation method, a pressure impregnation method, a
centrifugal impregnation method or the like may be used. Examples
of the exterior material of the closed-type nickel-metal hydride
rechargeable battery include those made of iron, iron subjected to
plating with a metal material such as nickel, stainless steel, a
polyolefin-based resin and so on.
The form of the closed-type nickel-metal hydride rechargeable
battery is not particularly limited, and examples thereof include
batteries including a positive electrode, a negative electrode and
a monolayer or multilayer separator, such as coin batteries, button
batteries, prismatic batteries and flat batteries, and cylindrical
batteries including a roll-shaped positive electrode, negative
electrode and separator.
(Second Embodiment)
The hydrogen-storing alloy according to the second embodiment of
the present invention is one represented by the general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dM.sub.c).sub.x (where
0.1<a<0.25; 0.1<b<0.2; 0.02<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.6<x<3.7; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co).
In the hydrogen storing alloy according to the second embodiment of
the present invention, B/A (numerical value range of x) is 3.6 or
more and 3.7 or less as shown in the above-mentioned general
formula. When a hydrogen storing alloy in which B/A falls within
the above-mentioned range, the content of a Pr.sub.5Co.sub.19 phase
and Ce.sub.5Co.sub.19 phase is 80% by mass or more, so that
corrosion resistance is improved. In either the case where B/A is
3.6 or less or the case where B/A is 3.7 or more, corrosion
resistance is deteriorated.
It has become apparent that when an alloy having a specific B/A
ratio as described above is substituted with a predetermined amount
of Sm (samarium), a hydrogen storing alloy having high corrosion
resistance (durability) can be obtained. For improving corrosion
resistance, the Sm substitution amount is 0.1 or more and 0.25 or
less in terms of a molar amount based on the total amount of Sm, RE
(at least one element selected from the group consisting of a rare
earth element other than Sm, and Y) and Mg. In either the case
where the Sm substitution amount is less than 0.1 or the case where
the Sm substitution amount is more than 0.25, corrosion resistance
is deteriorated.
For the mechanism in which corrosion resistance is improved when
the Sm substitution amount is 0.1 or more and 0.25 or less, it is
thought that when the alloy is substituted with a predetermined
amount of Sm, the A.sub.5B.sub.19 phase (Pr.sub.5Co.sub.19 phase
and Ce.sub.5Co.sub.19 phase) is stably generated, so that the
crystal structure is uniquely stabilized, and thus distortion of
the overall alloy can be prevented.
In the hydrogen storing alloy according to the second embodiment of
the present invention, RE is at least one element selected from the
group consisting of a rare earth element other than Sm, and Y, and
essentially contains La as a main component. The content of La is
preferably 0.6 or more, more preferably 0.7 or more in terms of a
molar amount based on the total amount of Sm, RE (at least one
element selected from the group consisting of a rare earth element
other than Sm, and Y) and Mg. Examples of RE excluding La include
Nd, Pr, Ce, Dy and Gd, and among them, Nd and/or Pr are preferred
because corrosion resistance is improved when these elements are
contained.
In the hydrogen storing alloy according to the second embodiment of
the present invention, the content of Mg is more than 0.1 and less
than 0.2 in terms of a molar amount based on the total amount of
Sm, RE (at least one element selected from the group consisting of
a rare earth element other than Sm, and Y) and Mg for improving
corrosion resistance. The content of Mg is preferably less than
0.17. In either the case where the Al amount is 0.1 or less or the
case where the Al amount is 0.2 or more, corrosion resistance is
deteriorated.
In the hydrogen storing alloy according to the second embodiment of
the present invention, the main component that forms B is Ni, and
when along with Ni, Al is contained in an amount of more than 0.02
and less than 0.2 in terms of molar amount multiplied by X, a
hydrogen storing alloy having improved corrosion resistance is
obtained. The content of Al is preferably more than 0.05. In either
the case where the Al amount is 0.02 or less or the case where the
Al amount is 0.2 or more, corrosion resistance is deteriorated.
Along with Al, Mn and/or Co can be contained in an amount of 0.1 or
less in terms of a molar amount multiplied by X for improving
corrosion resistance.
In the hydrogen storing alloy according to the second embodiment of
the present invention, corrosion resistance is improved when the
crystal phase contains 80% by mass or more of a Or.sub.5Co.sub.19
phase and a Ce.sub.5Co.sub.19 phase in total. The content of both
the phases is more preferably 90% or more. The content of the
Ce.sub.5Co.sub.19 is preferably 80% by mass or more. In addition to
the Pr.sub.5Co.sub.19 phase and Ce.sub.5Co.sub.19 phase, a
Ce.sub.2Ni.sub.7 phase, a Gd.sub.2Co.sub.7 phase, a CaCu.sub.5
phase or the like may be contained.
A method for producing the hydrogen storing alloy of this
embodiment includes, for example, a melting step of melting an
alloy raw material blended so as to achieve a predetermined
composition ratio as described above; a cooling step of solidifying
the molten alloy raw material; an annealing step of annealing the
cooled alloy; and a grinding step of grinding the alloy.
The steps will be described more in detail. First, a predetermined
amount of a raw material ingot (alloy raw material) is weighed
based on the chemical composition of an intended hydrogen storing
alloy.
In the melting step, the alloy raw material is added in crucible,
and melted at 1000.degree. C. or higher in an inert gas atmosphere
or in vacuum using a high-frequency melting furnace etc. The upper
limit of the melting temperature is about 2000.degree. C. For
example, the alloy raw material is heated at 1200 to 1600.degree.
C. to be melted.
In the cooling step, the molten alloy raw material is cooled to be
solidified. The cooling speed may be either a slow cooling speed or
1000 K/second or more (also referred to as a rapid cooling speed),
but it is preferred to use a rapid cooling speed. Rapid cooling at
1000 K/second has an effect of causing the alloy composition to
become fine and heterogeneous. The cooling speed can be set to
1000000 K/second or less.
Specifically, a water-cooling mold method, a melt spinning method
with a cooling speed of 100000 K/second or more, a gas atomizing
method with a cooling speed of about 10000 K/second, or the like
can be used as the cooling method.
In the annealing step, the alloy is heated at 860.degree. C. or
higher and 1000.degree. C. or lower in a compressed state under an
inert gas atmosphere using, for example, an electric furnace etc.
Preferably, the alloy is heated at 930 to 975.degree. C. The
compression condition is preferably 0.2 MPa (gauge pressure) or
more and 1.0 MPa (gauge pressure) or less. The treatment time is
the annealing step is preferably 3 hours or more and 50 hours or
less.
The grinding step may be carried out either before or after
annealing, but since the surface area is increased by grinding, it
is desirable to carry out the grinding step after the annealing
step for preventing oxidation of the surface of the alloy.
Preferably, grinding is performed in an inert atmosphere for
preventing oxidation of the surface of the alloy.
As grinding means, for example, mechanical grinding, hydrogenation
grinding or the like is used, and it is preferred to perform
grinding in such a manner that the particle size of hydrogen
storing alloy particles after grinding is approximately 20 to 70
.mu.m.
The application of the hydrogen storing alloy according to the
second embodiment of the present invention is not particularly
limited, and it can be used in various applications such as
nickel-metal hydride rechargeable batteries, fuel batteries, and
fuel tanks for hydrogen automobiles. Particularly, the hydrogen
storing alloy is suitably used for a negative active material of a
nickel-metal hydride rechargeable battery. A nickel-metal hydride
rechargeable battery including a negative electrode containing the
hydrogen storing alloy according to the second embodiment of the
present invention as described above is also one aspect of the
present invention.
The nickel-metal hydride rechargeable battery according to the
second embodiment of the present invention further includes a
positive electrode (nickel electrode) containing a positive active
material mainly composed of nickel hydroxide, a separator, an
alkali electrolyte solution and so on in addition to the negative
electrode containing as a negative active material the hydrogen
storing alloy according to the second embodiment of the present
invention.
The negative electrode contains as a negative active material the
hydrogen storing alloy according to the second embodiment of the
present invention. For example, the hydrogen storing alloy
according to the second embodiment of the present invention is
blended in the negative electrode in the form of a powdered
hydrogen storing alloy.
The negative electrode may contain a conducting agent, a binder
(including a thickener) and so on in addition to the hydrogen
storing alloy powder.
Examples of the conducting agent include carbon-based conducting
agents such as natural graphite (scaly graphite, scalelike, earthy
graphite and the like), artificial graphite, carbon black,
acetylene black, ketjen black, carbon whiskers, carbon fibers and
vapor phase growth carbon; and metal-based conducting agents
composed of powders, fibers and the like of metals such as nickel,
cobalt and copper. These conducting agents may be used alone, or
may be used in combination of two or more thereof. A rare earth
oxide such as yttrium oxide may be contained as an anticorrosive
agent.
The blending amount of the conducting agent is preferably 0.1 to 10
parts by mass, more preferably 0.2 to 5 parts by mass based on 100
parts by mass of the hydrogen storing alloy powder. When the
blending amount of the conducting agent is less than 0.1 part by
mass, it is difficult to achieve sufficient conductivity. On the
other hand, when the blending amount of the conducting agent is
more than 10 parts by mass, the discharge capacity improving effect
may be insufficient.
Examples of the binder include polyolefin-based resins such as
polytetrafluoroethylene (PTHE), polyethylene and polypropylene,
ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene
butadiene rubber, fluororubber, polyvinyl alcohol, methyl
cellulose, carboxymethyl cellulose and xanthan gum. These binders
may be used alone, or may be used in combination of two or more
thereof.
The blending amount of the binder is preferably 0.1 to 1.0 parts by
mass, more preferably 0.5 to 1.0 parts by mass based on 100 parts
by mass of the hydrogen storing alloy powder. When the blending
amount of the binder is less than 0.1 part by mass, it is difficult
to achieve sufficient thickenability. On the other hand, when the
blending amount of the binder is more than 1.0 part by mass, the
performance of the electrode may be deteriorated.
The positive electrode is, for example, an electrode containing as
a positive active material a nickel hydroxide composite oxide
formed by mixing zinc hydroxide or cobalt hydroxide with nickel
hydroxide as a main component. As the nickel hydroxide composite
oxide, one that is uniformly dispersed by a coprecipitation method
is suitably used.
Preferably, the positive electrode contains an additive for
improving electrode performance in addition to the nickel hydroxide
composite oxide. The additive is, for example, a conductivity
modifier such as cobalt hydroxide or cobalt oxide. Alternatively,
the nickel hydroxide composite oxide may be coated with cobalt
hydroxide, or the nickel hydroxide composite oxide may be partially
oxidized by oxygen, an oxygen-containing gas,
K.sub.2S.sub.2O.sub.8, hypochlorous acid or the like.
As the additive, a compound containing a rare earth element such as
Y or Yb, or an oxygen overvoltage improving substance such as a
compound containing Ca can also be used. The rare earth element
such as Y or Yb is partially dissolved and disposed on the surface
of the negative electrode, and is therefore expected to exhibit an
effect of suppressing corrosion of a negative active material.
The positive electrode may further contain the conducting agent,
binder and so on as in the case of the negative electrode.
The above-mentioned positive electrode and negative electrode can
be produced by, for example, adding the conducting agent, binder
and so on to each active material as necessary, then mixing the
mixture together with water or an organic solvent such as an
alcohol or toluene to obtain a paste, applying the paste to a
conductive support, drying the paste, and performing roll
molding.
Examples of the conductive support include steel plates, and plated
steel plates obtained by subjecting a steel plate to plating with a
metal material such nickel. Examples of the form of the conductive
support include foams, molded products of fiber groups,
three-dimensional substrates subjected to irregularity processing,
and two-dimensional substrates such as punching plates. Among these
conductive supports, a foam made from nickel excellent in corrosion
resistance to alkalis and resistance to oxidation and composed of a
porous structure that is a structure excellent in current
collection property is preferred as a conductive support for a
positive electrode. On the other hand, as a conductive support for
a negative electrode, a pierced steel plate obtained by subjecting
to nickel plating an iron foil which is inexpensive and excellent
in conductivity.
The thickness of the conductive support is preferably 30 to 100
.mu.m, more preferably 40 to 70 .mu.m. When the thickness of the
conductive support is less than 30 .mu.m, productivity may be
deteriorated. On the other hand, when the thickness of the
conductive support is more than 100 .mu.m, the discharge capacity
may be insufficient.
When the conductive support is porous, the inner diameter thereof
is preferably 0.8 to 2 .mu.m, more preferably 1 to 1.5 .mu.m. When
the inner diameter is less than 0.8 .mu.m, productivity may be
deteriorated. On the other hand, when the inner diameter is more
than 2 .mu.m, the retention capacity of the hydrogen storing alloy
may be insufficient.
Examples of the method for applying a paste for each electrode to
the conductive support include roller coating using an applicator
roll etc., screw coating, blade coating, spin coating and bar
coating.
Examples of the separator include a porous films and nonwoven
fabrics made from a polyolefin-based resin such as polyethylene or
polypropylene, an acryl, a polyamide or the like.
The weight per unit area of the separator is preferably 40 to 100
g/m.sup.2. When the weight per unit area is less than 40 g/m.sup.2,
a short circuit or deterioration of self discharge performance may
occur. On the other hand, when the weight per unit area is more
than 100 g/m.sup.2, the battery capacity tends to decrease because
the ratio of the separator per unit volume increases. The air
permeability of the separator is preferably 1 to 50 cm/sec. When
the air permeability is less than 1 cm/sec, the battery internal
pressure may be excessively high. On the other hand, when the air
permeability is more than 50 cm/sec, a short circuit or
deterioration of self discharge performance may occur. Further, the
average fiber diameter of the separator is preferably 1 to 20
.mu.m. When average fiber diameter is less than 1 .mu.m, the
strength of the separator may be reduced, leading to an increase in
defect rate in a battery assembling step. On the other hand, when
the average fiber diameter is more than 20 .mu.m, a short circuit
or deterioration of self discharge performance may occur.
Preferably, the separator is subjected to a hydrophilization
treatment at the fiber surface thereof. Examples of the
hydrophilization treatment include a sulfonation treatment, a
corona treatment, a fluorine gas treatment and a plasma treatment.
Particularly, a separator subjected to a sulfonation treatment at
the fiber surface is preferred because it has a high capability of
adsorbing impurities such as NO.sub.3.sup.-, NO.sub.2.sup.- and
NH.sub.3.sup.- and eluted elements from a negative electrode, which
cause a shuttle phenomenon, and therefore exhibits a high self
discharge suppressing effect.
Examples of the alkali electrolyte solution include alkaline
aqueous solutions containing potassium hydroxide, sodium hydroxide,
lithium hydroxide or the like. The alkali electrolyte solutions may
be used alone, or may be used in combination of two or more
thereof.
The concentration of the alkali electrolyte solution is preferably
9.0 M or less, more preferably 5.0 to 8.0 M in terms of the sum of
ion concentrations.
To the alkali electrolyte solution may be added various additives
for improvement of the oxygen overvoltage at the positive
electrode, improvement of corrosion resistance of the negative
electrode and improvement of self discharge. Examples of such
additives include oxides and hydroxides such as those of Y, Yb, Er,
Ca and Zn. These additives may be used alone, or may be used in
combination of two or more thereof.
In the case where the nickel-metal hydride rechargeable battery
according to the second embodiment of the present invention is an
open-type nickel-metal hydride rechargeable battery, the battery
can be produced by, for example, sandwiching a negative electrode
in a positive electrode with a separator interposed therebetween,
fixing the electrodes so as to apply a predetermined pressure to
the electrodes, injecting an alkali electrolyte solution in this
state, and assembling an open-type cell.
On the other hand, in the case where the nickel-metal hydride
rechargeable battery according to the second embodiment of the
present invention is a closed-type nickel-metal hydride
rechargeable battery, the battery can be produced by injecting an
alkali electrolyte solution before or after laminating a positive
electrode, a separator and a negative electrode, and sealing the
electrolyte solution with an exterior material. In a closed-type
nickel-metal hydride rechargeable battery formed by winding a power
generating element in which a positive electrode and a negative
electrode are laminated with a separator interposed therebetween,
it is preferred to inject an alkali electrolyte solution in the
power generating element before or after winding the power
generating element. The method for injecting an alkali electrolyte
solution is not particularly limited, and the alkali electrolyte
solution may be injected at normal pressure, or for example, a
vacuum impregnation method, a pressure impregnation method, a
centrifugal impregnation method or the like may be used. Examples
of the exterior material of the closed-type nickel-metal hydride
rechargeable battery include those made of iron, iron subjected to
plating with a metal material such as nickel, stainless steel, a
polyolefin-based resin and so on.
The form of the closed-type nickel-metal hydride rechargeable
battery is not particularly limited, and examples thereof include
batteries including a positive electrode, a negative electrode and
a monolayer or multilayer separator, such as coin batteries, button
batteries, prismatic batteries and flat batteries, and cylindrical
batteries including a roll-shaped positive electrode, negative
electrode and separator.
The present invention will now be described further in detail by
way of examples, but the present invention is not limited to these
examples.
EXAMPLES
(Examples According to the First Embodiment of the Present
Invention)
<Method for Preparing Hydrogen Storing Alloy>
A predetermined amount of a raw material ingot was weighed in such
a manner that the alloy had a composition in each of Examples 1 to
10 and Comparative Examples 1 to 9 as described in Table 1, and the
raw material ingot was added in a crucible, and heated to
1500.degree. C. under a decompressed argon gas atmosphere using a
high-frequency melting furnace, so that the raw material was
melted. After the raw material was melted, it was rapidly cooled at
500000 K/sec by using a melt spinning method, so that the alloy was
solidified.
Next, the obtained alloy was heat-treated for 5 hours under an
atmosphere of argon gas compressed to 0.2 MPa (gauge pressure, the
same hereinafter), and the obtained hydrogen storing alloy was
ground to obtain a hydrogen storing alloy powder having an average
particle size (D50) of 50 .mu.m.
The heat treatment was performed at 950.degree. C. for the hydrogen
storing alloy powders of Examples 1 to 9 and Comparative Examples 1
to 9, and at 930.degree. C. for the hydrogen storing alloy of
Example 10.
<Preparation of Open-type Nickel Hydrogen Battery>
3 parts by mass of a nickel powder (#210 manufactured by INCO
Limited) was added to and mixed with 100 parts by mass of the
hydrogen storing alloy powder prepared in the manner described
above, an aqueous solution with a thickener (methyl cellulose)
dissolved therein was then added, and 1 part by mass of a binder
(styrene butadiene rubber) was further added to form a paste. The
paste was applied to both surfaces of a 35 .mu.m-thick pierced
steel plate (opening ratio: 50%), and dried, and the coated plate
was then pressed to a thickness of 0.33 mm to obtain a negative
electrode plate.
For a positive electrode plate, a sintering nickel hydroxide
electrode having a capacity three times as large as the negative
electrode capacity.
The negative electrode plate prepared in the manner described above
was sandwiched in the positive electrode plate with a separator
interposed therebetween to form electrodes, a 7 M aqueous potassium
hydroxide solution was injected with these electrodes fixed in such
a manner as to apply a pressure of 1 kgf to the electrodes, and an
open-type cell was assembled.
<Evaluation of Open-Type Nickel Hydrogen Battery>
A cycle was repeated 10 times in which the open-type nickel
hydrogen battery prepared in the manner described above was charged
to 150% at 0.1 ItA (31 mA/g) and discharged to a negative electrode
final potential of -0.6 V (to Hg/HgO) at 0.2 ItA in a water bath at
20.degree. C. Further a cycle was repeated 40 times in which the
open-type nickel hydrogen battery was charged to 105% at 1 ItA and
discharged to a negative electrode final potential of -0.6 V (to
Hg/HgO) at 1 ItA. Under these conditions, total 50 cycles of
charge-discharge were performed.
<Measurement of Specific Surface Area)
Measurement of the specific surface area of the alloy before and
after the cycle test was performed in accordance with the following
procedure. The negative electrode plate was taken out from the
battery before and after each of the 40 cycles of charge-discharge,
washed with water, and dried, and a mixture layer portion and a
base plate (a punching steel plate with iron subjected to nickel
plating) were then separated from each other. Next, the mixture
layer portion was ground with a mortar, and put in a specific
surface area measurement apparatus (MONOSORB manufactured by
QUANTACHROME Co.), and the specific surface area was measured by a
BET method.
<Analysis of Crystal Phase>
Analysis of the crystal phase was performed in accordance with the
following procedure. X-ray diffraction measurement of the alloy
powder was performed using a powder X-ray diffractometer (Rigaku
MinifulexII). Cu was used for an X-ray tube, an accelerating
voltage of 30 kV and a current of 15 mA were set as power, and
Cu-K.alpha. was used. Step scanning with an integral time of 2
seconds at intervals of 0.02 was performed over a measurement range
of 2.theta.=5-90.degree.. The powder X-ray diffraction pattern
obtained in the above-mentioned measurement was analyzed by a
Rietveld method (analysis software: RIETAN-2000) to calculate an
abundance ratio (% by mass) of the crystal phase contained in the
alloy.
Next, in the following manner, a closed-type battery for a
charge-discharge cycle test was prepared, and a charge-discharge
cycle test was conducted.
<Preparation of Positive Electrode Plate for Nickel-Metal
Hydride Rechargeable Battery>
The surface of nickel hydroxide containing 3% by mass of zinc and
0.6% by mass of cobalt in a solid solution state was coated with a
7% by mass of cobalt hydroxide, and then subjected to an air
oxidation treatment at 110.degree. C. for 1 hour using a 18 M
sodium hydroxide solution. The material thus obtained was used as a
positive active material. Further, 2% by mass of Yb.sub.2O.sub.3
was added to and mixed with the positive active material, an
aqueous solution with a thickener (carboxymethyl cellulose)
dissolved therein was then added to prepare a paste, and foamed
nickel having a substrate surface density of 300 g/m.sup.2 was
filled with the paste, dried, and the pressed to a predetermined
thickness to prepare a positive electrode plate having a capacity
of 2000 mAh.
<Preparation of Negative Electrode Plate for Nickel-Metal
Hydride Rechargeable Battery>
An aqueous solution with a thickener (methyl cellulose) dissolved
therein was added to 100 parts by mass of the hydrogen storing
alloy powder of each of Examples 1 to 10 and Comparative Examples 1
to 9, which was ground so as to have an average particle size D50
of 50 .mu.m, and 1 part by mass of a binder (styrene butadiene
rubber) was further added to form a paste. The paste was applied to
both surfaces of a 35 .mu.m-thick pierced steel plate, and dried,
and the coated plate was then pressed to a predetermined thickness
to prepare a negative electrode plate having a capacity of 2600
mAh.
<Preparation of Closed-type Nickel-Metal Hydride Rechargeable
Battery>
A separator (120 .mu.m-thick polypropylene nonwoven fabric) was
folded in two at substantially the center in the longitudinal
direction, the positive electrode plate was sandwiched therein, the
negative electrode plate containing the hydrogen storing alloy
powder of each of Examples 1 to 10 and Comparative Examples 1 to 9
was superimposed on the outer side, and the layered product thus
obtained was wound in such a manner as to situate the negative
electrode plate on the outer peripheral side, so that an electrode
group was formed. The obtained electrode group was housed in
cylindrical metallic battery case, 2.6 g of an electrolyte solution
containing 5 M KOH, 3 M NaOH and 0.8 M LiOH was then injected
therein, and the battery case was capped with a metallic lid
provided with a safety valve, so that an AA size nickel-metal
hydride rechargeable battery having a capacity of 2100 mAh was
prepared as a test battery.
<Initial Formation>
For each test battery, initial formation was performed in
accordance with the following procedure. A cycle was repeated twice
in which the battery was charged at 200 mA for 16 hours, and then
discharged to 1 V at 400 mA at 20.degree. C. Thereafter, the
battery was stored at 40.degree. C. for 48 hours. A cycle was then
repeated twice in which the battery was charged at 20.degree. C. at
200 mA for 16 hours, rested for 1 hour, and discharged to 1 V at
400 mA, thus completing formation.
<Charge-Discharge Cycle Test>
The charge-discharge cycle test was conducted with the battery
charged at -dV=5 mV at 0.5 ItA, then rested for 30 minutes, and
then discharged at 1 V (20.degree. C.) at 0.5 ItA. The time at
which the discharge capacity decreased to 60% of the initial
capacity was judged as the end of the cycle life.
The alloy compositions of the hydrogen storing alloys of Examples 1
to 10 and Comparative Examples 1 to 9, and also the results of
measurement of the abundance ratio of the crystal phase, the
specific surface area, and the cycle life of the nickel-metal
hydride rechargeable battery using the hydrogen storing alloy are
shown in Table 1.
TABLE-US-00001 TABLE 1 Alloy composition B/A RATIO RE ATOM M ATOM
(GENERAL La Nd Pr Y Sm Mg Ni Al Mn CO FORMULA: X) Example 1 0.4 0 0
0.49 0.11 3.26 0.16 0 0 3.42 Example 2 0.5 0 0 0.39 0.11 3.26 0.16
0 0 3.42 Example 3 0.4 0 0 0.49 0.11 3.16 0.16 0 0 3.32 Example 4
0.3 0 0 0.59 0.11 3.26 0.16 0 0 3.42 Example 5 0.3 0.1 0 0.49 0.11
3.26 0.16 0 0 3.42 Example 6 0.3 0 0.1 0.49 0.11 3.26 0.16 0 0 3.42
Example 7 0.3 0 0 0.1 0.49 0.11 3.26 0.16 0 0 3.42 Example 8 0.4 0
0 0.49 0.11 3.21 0.16 0.1 0 3.42 Example 9 0.4 0 0 0.49 0.11 3.16
0.16 0 0.1 3.42 Example 10 0.4 0 0 0.49 0.11 3.26 0.16 0 0 3.42
Comparative 0.84 0 0 0 0.16 3.26 0.16 0 0 3.42 Example 1
Comparative 0.29 0 0 0.6 0.11 3.26 0.16 0 0 3.42 Example 2
Comparative 0.6 0 0 0.29 0.11 3.26 0.16 0 0 3.42 Example 3
Comparative 0.35 0 0 0.49 0.16 3.26 0.16 0 0 3.42 Example 4
Comparative 0.4 0 0 0.49 0.11 3.34 0.08 0 0 3.42 Example 5
Comparative 0.4 0 0 0.49 0.11 3.21 0.21 0 0 3.42 Example 6
Comparative 0.5 0 0 0.39 0.11 3.39 0.16 0 0 3.55 Example 7
Comparative 0.4 0 0 0.49 0.11 2.99 0.16 0 0 3.15 Example 8
Comparative 0 0.4 0 0.49 0.11 3.26 0.16 0 0 3.42 Example 9 Specific
Abundance ratio of crystal phase (% by mass) surface Cycle
Ce.sub.2Ni.sub.7 Gd.sub.2Co.sub.7 Pr.sub.5Co.sub.19
Ce.sub.5Co.sub.19 CaC- u.sub.5 Others area (m.sup.2/g) life Example
1 74 0 15 6 4 1 1.52 190 Example 2 75 0 11 5 2 7 1.64 180 Example 3
81 0 10 6 2 1 1.63 190 Example 4 69 0 14 12 4 1 1.68 170 Example 5
76 0 10 9 4 1 1.5 200 Example 6 71 0 18 6 4 1 1.49 200 Example 7 83
0 8 6 2 1 1.52 190 Example 8 72 0 15 7 5 1 1.62 180 Example 9 73 0
15 6 4 2 1.48 190 Example 10 60 0 19 16 4 1 1.62 170 Comparative 70
0 23 5 2 0 2.46 50 Example 1 Comparative 68 0 32 0 0 0 1.91 100
Example 2 Comparative 50 5 21 15 5 4 1.81 110 Example 3 Comparative
70 0 14 1 15 0 2.27 50 Example 4 Comparative 75 0 10 4 2 9 1.95 80
Example 5 Comparative 65 0 17 6 12 0 2.23 70 Example 6 Comparative
54 0 31 11 2 2 1.85 100 Example 7 Comparative 67 0 5 2 10 16 2.15
60 Example 8 Comparative 71 0 20 5 2 2 1.8 90 Example 9
When corroded, the hydrogen storing alloy morphologically changes
with a hydroxide of rare earth etc. generated on the surface. Thus,
a specific surface area value can be determined and used as an
index of the alloy corrosion level. A smaller specific surface area
value may correspond to a lower corrosion level, while a larger
specific surface area value may correspond to a high corrosion
level. From Table 1, it is apparent that the hydrogen storing
alloys of Examples 1 to 10 which are represented by the general
formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.3<a<0.6; 0<b<0.16; 0.1<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.2<x<3.5; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or Co)
have a smaller specific surface area value and a lower corrosion
level as compared to the hydrogen storing alloy in which a (Sm
substitution amount) is 0.3 or less (Comparative Examples 1 and 3)
or 0.6 or more (Comparative Example 2), the hydrogen storing alloy
in which b is 0.16 or more (Comparative Example 4), the hydrogen
storing alloy in which cx is 0.1 or less (Comparative Example 5) or
0.2 or more (Comparative Example 6), the hydrogen storing alloy in
which x (B/A ratio) is 3.5 or more (Comparative Example 7) or 3.2
or less (Comparative Example 8), and the hydrogen absorbing alloy
which does not contain La as a RE atom (Comparative Example 9). The
specific surface area value is preferably 1.7 m.sup.2/g or less,
more preferably 1.60 m.sup.2/g or less.
From Table 1, it is apparent that the hydrogen storing alloys of
Examples 1 to 10 which have a small specific surface area have
improved corrosion resistance (durability) as compared to the
hydrogen storing alloys of Comparative Examples 1 to 9 which have a
large specific surface area, and the nickel-metal hydride
rechargeable batteries obtained using the hydrogen storing alloys
of Examples 1 to 10 have a remarkably improved cycle life as
compared to the nickel-metal hydride rechargeable batteries
obtained using the hydrogen storing alloys of Comparative Examples
1 to 9. Particularly, the hydrogen storing alloy in which as RE, La
is contained in an amount of 0.3 or more in terms of a molar amount
based on the total amount of RE, Sm and Mg, and also Nd and/or Pr
are used in combination (Examples 5 and 6) has a small specific
surface area, and is excellent in corrosion resistance
(durability), and the nickel-metal hydride rechargeable battery
obtained using such a hydrogen storing alloy has an improved cycle
life.
When among the hydrogen storing alloys of Examples 1 to 10 which
have a small specific surface area, the hydrogen storing alloys of
Examples 1 to 3 and 5 to 9 which contain 70% by mass or more of a
Ce.sub.2Ni.sub.7 phase as a crystal phase of the alloy are used,
corrosion resistance (durability) is further improved, and the
cycle life of the nickel-metal hydride rechargeable battery is
improved.
(Examples According to the Second Embodiment of the Present
Invention)
<Method for Preparing Hydrogen Storing Alloy>
A predetermined amount of a raw material ingot was weighed in such
a manner that the alloy had a composition in each of Examples 1 to
13 and Comparative Examples 1 to 8 as described in Table 2, and the
raw material ingot was added in a crucible, and heated to
1500.degree. C. under a decompressed argon gas atmosphere using a
high-frequency melting furnace, so that the raw material was
melted. After the raw material was melted, it was rapidly cooled at
500000 K/sec by using a melt spinning method, so that the alloy was
solidified.
Next, the obtained alloy was heat-treated for 5 hours under an
atmosphere of argon gas compressed to 0.2 MPa (gauge pressure, the
same hereinafter), and the obtained hydrogen storing alloy was
ground to obtain a hydrogen storing alloy powder having an average
particle size (D50) of 50 .mu.m.
The heat treatment was performed at 950.degree. C. for the hydrogen
storing alloy powders of Examples 1 to 12 and Comparative Examples
1 to 8, and at 930.degree. C. for the hydrogen storing alloy of
Example 13.
<Preparation of Open-type Nickel Hydrogen Battery>
3 parts by mass of a nickel powder (#210 manufactured by INCO
Limited) was added to and mixed with 100 parts by mass of the
hydrogen storing alloy powder prepared in the manner described
above, an aqueous solution with a thickener (methyl cellulose)
dissolved therein was then added, and 1 part by mass of a binder
(styrene butadiene rubber) was further added to form a paste. The
paste was applied to both surfaces of a 35 .mu.m-thick pierced
steel plate (opening ratio: 50%), and dried, and the coated plate
was then pressed to a thickness of 0.33 mm to obtain a negative
electrode plate.
For a positive electrode plate, a sintering nickel hydroxide
electrode having a capacity three times as large as the negative
electrode capacity.
The negative electrode plate prepared in the manner described above
was sandwiched in the positive electrode plate with a separator
interposed therebetween to form electrodes, a 7 M aqueous potassium
hydroxide solution was injected with these electrodes fixed in such
a manner as to apply a pressure of 1 kgf to the electrodes, and an
open-type cell was assembled.
<Evaluation of Open-type Nickel Hydrogen Battery>
A cycle was repeated 10 times in which the open-type nickel
hydrogen battery prepared in the manner described above was charged
to 150% at 0.1 ItA (31 mA/g) and discharged to a negative electrode
final potential of -0.6 V (to Hg/HgO) at 0.2 ItA in a water bath at
20.degree. C. Further a cycle was repeated 40 times in which the
open-type nickel hydrogen battery was charged to 105% at 1 ItA and
discharged to a negative electrode final potential of -0.6 V (to
Hg/HgO) at 1 ItA. Under these conditions, total 50 cycles of
charge-discharge were performed.
<Measurement of Specific Surface Area)
Measurement of the specific surface area of the alloy before and
after the cycle test was performed in accordance with the following
procedure. The negative electrode plate was taken out from the
battery before and after each of the 40 cycles of charge-discharge,
washed with water, and dried, and a mixture layer portion and a
base plate (a punching steel plate with iron subjected to nickel
plating) were then separated from each other. Next, the mixture
layer portion was ground with a mortar, and put in a specific
surface area measurement apparatus (MONOSORB manufactured by
QUANTACHROME Co.), and the specific surface area was measured by a
BET method.
<Analysis of Crystal Phase>
Analysis of the crystal was performed in accordance with the
following procedure. X-ray diffraction measurement of the alloy
powder was performed using a powder X-ray diffractometer (Rigaku
MinifulexII). Cu was used for an X-ray tube, an accelerating
voltage of 30 kV and a current of 15 mA were set as power, and
Cu-K.alpha. was used. Step scanning with an integral time of 2
seconds at intervals of 0.02 was performed over a measurement range
of 2.theta.=5-90.degree.. The powder X-ray diffraction pattern
obtained in the above-mentioned measurement was analyzed by a
Rietveld method (analysis software: RIETAN-2000) to calculate an
abundance ratio (% by mass) of the crystal phase contained in the
alloy.
Next, in the following manner, a closed-type battery for a
charge-discharge cycle test was prepared, and a charge-discharge
cycle test was conducted.
<Preparation of Positive Electrode Plate for Nickel-Metal
Hydride Rechargeable Battery>
The surface of nickel hydroxide containing 3% by mass of zinc and
0.6% by mass of cobalt in a solid solution state was coated with a
7% by mass of cobalt hydroxide, and then subjected to an air
oxidation treatment at 110.degree. C. for 1 hour using a 18 M
sodium hydroxide solution. The material thus obtained was used as a
positive active material. Further, 2% by mass of Yb.sub.2O.sub.3
was added to and mixed with the positive active material, an
aqueous solution with a thickener (carboxymethyl cellulose)
dissolved therein was then added to prepare a paste, and foamed
nickel having a substrate surface density of 300 g/m.sup.2 was
filled with the paste, dried, and the pressed to a predetermined
thickness to prepare a positive electrode plate having a capacity
of 2000 mAh.
<Preparation of Negative Electrode Plate for Nickel-Metal
Hydride Rechargeable Battery>
An aqueous solution with a thickener (methyl cellulose) dissolved
therein was added to 100 parts by mass of the hydrogen storing
alloy powder of each of Examples 1 to 13 and Comparative Examples 1
to 8, which was ground so as to have an average particle size D50
of 50 .mu.m, and 1 part by mass of a binder (styrene butadiene
rubber) was further added to form a paste. The paste was applied to
both surfaces of a 35 .mu.m-thick pierced steel plate, and dried,
and the coated plate was then pressed to a predetermined thickness
to prepare a negative electrode plate having a capacity of 2600
mAh.
<Preparation of Closed-Type Nickel-Metal Hydride Rechargeable
Battery>
A separator (120 .mu.m-thick polypropylene nonwoven fabric) was
folded in two at substantially the center in the longitudinal
direction, the positive electrode plate was sandwiched therein, the
negative electrode plate containing the hydrogen storing alloy
powder of each of Examples 1 to 13 and Comparative Examples 1 to 8
was superimposed on the outer side, and the layered product thus
obtained was wound in such a manner as to situate the negative
electrode plate on the outer peripheral side, so that an electrode
group was formed. The obtained electrode group was housed in
cylindrical metallic battery case, 2.6 g of an electrolyte solution
containing 5 M KOH, 3 M NaOH and 0.8 M LiOH was then injected
therein, and the battery case was capped with a metallic lid
provided with a safety valve, so that an AA size nickel-metal
hydride rechargeable battery having a capacity of 2100 mAh was
prepared as a test battery.
<Initial Formation>
For each test battery, initial formation was performed in
accordance with the following procedure. A cycle was repeated twice
in which the battery was charged at 200 mA for 16 hours, and then
discharged to 1 V at 400 mA at 20.degree. C. Thereafter, the
battery was stored at 40.degree. C. for 48 hours. A cycle was then
repeated twice in which the battery was charged at 20.degree. C. at
200 mA for 16 hours, rested for 1 hour, and discharged to 1 V at
400 mA, thus completing formation.
<Charge-Discharge Cycle Test>
The charge-discharge cycle test was conducted with the battery
charged at -dV=5 mV at 0.5 ItA, then rested for 30 minutes, and
then discharged at 1 V (20.degree. C.) at 0.5 ItA. The time at
which the discharge capacity decreased to 60% of the initial
capacity was judged as the end of the cycle life.
The alloy compositions of the hydrogen storing alloys of Examples 1
to 13 and Comparative Examples 1 to 8, and also the results of
measurement of the abundance ratio of the crystal phase, the
specific surface area, and the cycle life of the nickel-metal
hydride rechargeable battery using the hydrogen storing alloy are
shown in Table 2.
TABLE-US-00002 TABLE 2 Alloy composition M RE ATOM ATOM B/A ratio
(general La Nd Pr Y Sm Mg Ni Al Mn CO formula: X) Example 1 0.72 0
0 0.13 0.15 3.48 0.15 0 0 3.63 Example 2 0.75 0 0 0.1 0.15 3.48
0.15 0 0 3.63 Example 3 0.6 0 0 0.25 0.15 3.48 0.15 0 0 3.63
Example 4 0.76 0 0 0.13 0.11 3.48 0.15 0 0 3.63 Example 5 0.72 0 0
0.13 0.15 3.53 0.1 0 0 3.63 Example 6 0.72 0 0 0.13 0.15 3.55 0.15
0 0 3.7 Example 7 0.72 0 0 0.13 0.15 3.45 0.15 0 0 3.6 Example 8
0.62 0.1 0 0.13 0.15 3.48 0.15 0 0 3.63 Example 9 0.62 0 0.1 0.13
0.15 3.48 0.15 0 0 3.63 Example 10 0.62 0 0 0.1 0.13 0.15 3.48 0.15
0 0 3.63 Example 11 0.72 0 0 0.13 0.15 3.48 0.1 0.1 0 3.63 Example
12 0.72 0 0 0.13 0.15 3.48 0.1 0 0.1 3.63 Example 13 0.72 0 0 0.13
0.15 3.48 0.15 0 0 3.63 Comparative 0.77 0 0 0.08 0.15 3.48 0.15 0
0 3.63 Example 1 Comparative 0.57 0 0 0.28 0.15 3.48 0.15 0 0 3.63
Example 2 Comparative 0.79 0 0 0.13 0.08 3.48 0.15 0 0 3.63 Example
3 Comparative 0.67 0 0 0.13 0.2 3.48 0.15 0 0 3.63 Example 4
Comparative 0.72 0 0 0.13 0.15 3.61 0.02 0 0 3.63 Example 5
Comparative 0.72 0 0 0.13 0.15 3.43 0.2 0 0 3.63 Example 6
Comparative 0.72 0 0 0.13 0.15 3.4 0.15 0 0 3.55 Example 7
Comparative 0.72 0 0 0.13 0.15 3.58 0.15 0 0 3.73 Example 8
Specific surface Abundance ratio of crystal phase (% by mass) area
Cycle Ce.sub.2Ni.sub.7 Gd.sub.2Co.sub.7 Pr.sub.5Co.sub.19
Ce.sub.5Co.sub.19 CaC- u.sub.5 Others Total (m.sup.2/g) life
Example 1 0 0 5 92 3 0 100 1.33 200 Example 2 0 0 7 89 3 1 100 1.35
190 Example 3 13 0 15 67 4 1 100 1.42 160 Example 4 0 0 7 90 3 0
100 1.31 180 Example 5 0 0 4 95 1 0 100 1.25 200 Example 6 0 0 6 94
0 0 100 1.41 170 Example 7 0 0 4 95 1 0 100 1.33 190 Example 8 0 0
7 90 3 0 100 1.23 210 Example 9 0 0 6 90 4 0 100 1.22 210 Example
10 10 0 1 85 4 0 100 1.45 160 Example 11 0 0 7 82 11 0 100 1.47 160
Example 12 0 0 5 92 3 0 100 1.29 200 Example 13 6 0 11 80 2 1 100
1.45 170 Comparative 0 0 20 72 7 1 100 1.56 100 Example 1
Comparative 20 0 23 40 15 2 100 1.59 90 Example 2 Comparative 0 0 4
95 1 0 100 1.67 80 Example 3 Comparative 10 0 15 53 18 4 100 1.65
80 Example 4 Comparative 5 1 15 65 12 2 100 1.74 80 Example 5
Comparative 0 0 21 47 18 14 100 1.58 90 Example 6 Comparative 21 0
27 42 9 1 100 1.61 90 Example 7 Comparative 0 0 18 65 16 1 100 1.68
90 Example 8
When corroded, the hydrogen storing alloy morphologically changes
with a hydroxide of rare earth etc. generated on the surface. Thus,
a specific surface area value can be determined and used as an
index of the alloy corrosion level. A smaller specific surface area
value may correspond to a lower corrosion level, while a larger
specific surface area may correspond to a high corrosion level. A
hydrogen storing alloy represented by the general formula:
(RE.sub.1-a-bSm.sub.aMg.sub.b)(Ni.sub.1-c-dAl.sub.cM.sub.d).sub.x
(where 0.1<a<0.25; 0.1<b<0.2; 0.02<cx<0.2;
0.ltoreq.dx.ltoreq.0.1; 3.6<x<3.7; RE is at least one element
selected from the group consisting of a rare earth element other
than Sm, and Y, and essentially contains La; and M is Mn and/or
Co). It is apparent that the hydrogen storing alloys of Examples 1
to 13 have a smaller specific surface area value and a lower
corrosion level as compared to the hydrogen storing alloy in which
a (Sm substitution amount) is less than 0.1 (Comparative Examples
1) or more than 0.25 (Comparative Example 2), the hydrogen storing
alloy in which b is 0.1 or less (Comparative Example 3) or 0.2 or
more (Comparative Example 4), the hydrogen storing alloy in which
cx is 0.02 or less (Comparative Example 5) or 0.2 or more
(Comparative Example 6), the hydrogen storing alloy in which x (B/A
ratio) is less than 3.6 (Comparative Example 7) or more than 3.7
(Comparative Example 8). The specific surface area value is
preferably 1.5 m.sup.2/g or less, more preferably 1.40 m.sup.2/g or
less.
From Table 2, it is apparent that the hydrogen storing alloys of
Examples 1 to 13 which have a small specific surface area have
improved corrosion resistance (durability) as compared to the
hydrogen storing alloys of Comparative Examples 1 to 8 which have a
large specific surface area, and the nickel-metal hydride
rechargeable batteries obtained using the hydrogen storing alloys
of Examples 1 to 13 have a remarkably improved cycle life as
compared to the nickel-metal hydride rechargeable batteries
obtained using the hydrogen storing alloys of Comparative Examples
1 to 8. Particularly, the hydrogen storing alloy in which as RE, La
is contained in an amount of 0.6 or more in terms of a molar amount
based on the total amount of RE, Sm and Mg, and also Nd and/or Pr
are used in combination (Examples 8 and 9) or the hydrogen storing
alloy containing Co as an M atom (Example 12) has a small specific
surface area, and is excellent in corrosion resistance
(durability), and the nickel-metal hydride rechargeable battery
obtained using such a hydrogen storing alloy has an improved cycle
life.
The hydrogen storing alloys of Examples 1 to 13 have a small
specific surface area, and has improved corrosion resistance
(durability) because they contain 80% by mass of a
Pr.sub.5Co.sub.19 phase and Ce.sub.5Co.sub.19 phase as a crystal
phase of the alloy, and thus the cycle life of the nickel-metal
hydride rechargeable battery is improved. When the hydrogen storing
alloy containing 90% by mass or more of both the phases (Examples
1, 2, 4 to 9, 12 and 13) is used, the cycle life of the
nickel-metal hydride rechargeable battery is further improved.
According to the present invention, a hydrogen storing alloy having
high corrosion resistance (durability), and therefore when the
hydrogen storing alloy is used as a negative electrode of a
nickel-metal hydride rechargeable battery, a nickel-metal hydride
rechargeable battery excellent in cycle life can be provided.
* * * * *